Catalysts for purification of exhaust gas have been conventionally used for removing harmful components such as hydrocarbon gas (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas from automotive engines. As such catalysts for purification of exhaust gas, there are known three-way catalysts capable of simultaneously cleaning HC, CO and NOx in exhaust gas combusted in the theoretical air-fuel ratio. Such a three-way catalyst has a nature of storing sulfur (S) components contained in exhaust gas in the form of sulfate during the use. Accordingly, when a three-way catalyst is used with a fuel containing a high concentration of sulfur components, the sulfur components are gradually accumulated therein (sulfur poisoning). In the meanwhile, the accumulated sulfur components are desorbed in a reduction atmosphere, and react with hydrogen in the exhaust gas to generate hydrogen sulfide (H2S) which gives an odor (H2S odor). Hence, studies on three-way catalysts have been conducted to reduce the generation of H2S, and various catalysts have been disclosed.
For example, Japanese Unexamined Patent Application Publication No. 2006-326495 (Document 1) discloses a catalyst for purification of exhaust gas comprising a metal oxide support, a noble metal, and nickel oxide (NiO). Such a catalyst for purification of exhaust gas traps a sulfur component in a way that NiO forms sulfide in a high-temperature reduction atmosphere, thereby suppressing the generation of H2S. Thus, the catalyst shows an excellent H2S-reducing effect. However, the catalyst for purification of exhaust gas as described in Document 1 has a problem that NiO, which is an environmentally hazardous substance, has to be used. Additionally, under high load conditions where the driving condition rapidly changes, such as hill-climbing or high-speed driving following low-speed driving on a city road, the amount of H2S emitted is not always sufficiently reduced with such a catalyst for purification of exhaust gas.
Meanwhile, Document 2 (SAE Paper Number 900611 [N.E. CHEMCAT Corporation]) discloses a catalyst for purification of exhaust gas, in which 0.03 mol/L-cat of Mo is supported on a catalyst formed by supporting platinum and rhodium (noble metals) on ceria (metal oxide support). However, although there is a report that such a catalyst for purification of exhaust gas described in Document 2 reduced the amount of H2S emitted into ¼, the ranges of added amounts necessary for the balance of the three-way activities are not explicitly shown.
Incidentally, Japanese Unexamined Patent Application Publication No. 2004-167354 (Document 3) discloses a catalyst for purification of exhaust gas comprising a composite oxide obtained by mixing Al2O3 and an acidic oxide made of at least one of ZrO2 and TiO2 in a primary-particle level; a noble metal supported on the composite oxide; and phosphorus added to the composite oxide. However, such a catalyst for purification of exhaust gas cannot always sufficiently reduce the amount of H2S emitted. In addition, such a catalyst for purification of exhaust gas does not function as a three-way catalyst, and the catalyst has to contain a basic substance or the like in order to function as a three-way catalyst.